Demand for cellular services is increasing rapidly as new broadband mobile services come on-line and are rapidly adopted. This leads to a preference for increased bandwidth capacity within cells and increased density of cells. The density of base station increases, both by reducing cell size and increasing the transmission bandwidth per cell, which is achieved by higher bandwidth throughput per carrier and using multiple carriers in each cell site.
This increase in base station density leads to a preference for small, simple, and low power wireless base stations at the antenna sites. Base station functions may be centralized to simplify the antenna site functions. In an example, the entire base station, except for a radio frequency (RF) transmitter and receiver, are centralized, with the in-phase (I) and quadrature (Q) components of the digital RF modem output being transmitted digitally over a low delay, low jitter digital optical path to the antenna site. At the antenna site, digital-to-analog (D/A) converters convert the digital streams into the I and Q components of the signal, modulated on to the outgoing RF carriers generated on site. These components are then fed via a power amplifier (PA) to the antenna for transmission. In the reverse direction, the output from the RF receiver at the antenna site may be oversampled, digitized using an analog-to-digital converter (A/D), and transmitted digitally back to a central location for digital I and Q extraction. The optical transmission link has a high bandwidth and a low consistent delay to avoid affecting the radio timing and modem performance, because excess delay, jitter, or errors from the transmission path are effectively injected into the modem process. Otherwise severe impairments in modem characteristics and RF characteristics may be seen.
A radio network, like other access networks, is subject to changing traffic patterns and traffic loads, as well as occasional equipment failures. Adding switching may accommodate the ability to reconfigure connections and capacities to antenna sites. However, the addition of switching may add undesirable latency (delay) and jitter to the connections from the central location to the antenna sites. Delay can have the effect of modifying the timing for round trip responses from user equipments, thereby reducing the maximum cell size, while jitter inserted into the middle of a modem can create major impairments in that modem.